Various semiconductor fabrication processes, such as plasma-assisted etching or chemical vapor deposition, are performed in plasma chambers in which a semiconductor workpiece 20 is mounted on a metal electrode 22 (see FIGS. 1 and 2). When the workpiece 20 is a circular semiconductor wafer, the cathode 22 generally has a circular top surface on which the wafer rests. Generally, a mixture of process reagent gases is supplied to the chamber while a pump maintains a vacuum inside the chamber. An electrical power source excites the process gas mixture to a plasma state. Typically, a radio frequency (RF) power supply 24 is capacitively coupled to the electrode 22 so as to produce on the electrode a negative bias voltage relative to the plasma body. The bias voltage attracts ions to bombard the workpiece so as to promote the desired fabrication process. Because it is negatively biased, the electrode 22 often is called the cathode electrode or cathode.
One objective in designing a plasma process chamber is to maximize the reaction rate of the plasma-enhanced process being performed in the chamber. The process rate will be undesirably reduced to the extent any portion of the ion flux from the plasma to the cathode bombards exposed portions of the cathode rather than the workpiece. Accordingly, to concentrate the RF current flow toward the workpiece 20, it is conventional to cover the side of the cathode 22 with a dielectric side shield 28 which is thick enough to present a high electrical impedance to RF current flow between the plasma and the side of the cathode.
In many conventional plasma chambers, the cathode 22 has a substantially larger diameter than the workpiece. To prevent RF current flow between the plasma and the portion of the cathode outside the perimeter of the workpiece, that portion of the cathode conventionally is covered by a dielectric top shield or collar 30. Like the side shield 28, the top shield 30 must sufficiently thick so that its electrical impedance reduces to a negligible level the RF current flow between the plasma and the portion of the cathode outside the perimeter of the workpiece.
One problem with conventional dielectric shields 28, 30 is that, depending on the process chemistry, exposed surfaces of the top shield 30 may be eroded by some of the chemical species present in the plasma, so that the top shield must be replaced periodically. In chambers lacking a top shield 30, side shield 28 may be exposed to the plasma, so that it will suffer the same erosion problem. Frequent replacement is undesirable because it requires suspending the production line while the chamber is shut down. The erosion of the dielectric shield may be especially severe in processes for etching dielectric layers on semiconductor workpieces, because the etchant species which etch the dielectric layer also may etch the dielectric collar.
Another objective in designing a semiconductor process plasma chamber is achieving spatial uniformity of the fabrication process over the surface of the workpiece. For example, in reactive ion etch processes and chemical vapor deposition processes, the process rate (i.e., the etch rate and deposition rate, respectively) may be slower in the center of the workpiece than at the periphery because the reactive species are more depleted near the center of the workpiece then near the periphery. In other words, such a process suffers from radial non-uniformity.
One conventional method of improving the spatial uniformity in the radial dimension is to surround the perimeter of the workpiece with an elevated cylindrical collar or shroud, sometimes called a focus ring. The elevated collar produces at least three effects, the first two of which typically reduce the process rate near the perimeter of the wafer. One effect of the elevated collar or shroud is that it obstructs reactive process gases outside the collar from travelling toward the wafer, so that the collar increases the depletion of reactive species near the wafer perimeter to more closely match the depletion near the wafer center. Another effect of the elevated collar is that it displaces axially upward the plasma sheath outside the workpiece perimeter, thereby moving the plasma sheath further from the workpiece perimeter, and consequently reducing the reactive species concentration near the perimeter of the workpiece. A third effect is that the elevated collar increases the residence time of reactive species near the perimeter of the wafer, which may either increase or decrease the process rate near the wafer perimeter, depending on the chemistry of the particular process being performed.
The elevated collar or shroud need not be a dielectric material to achieve the effects just described. However, if the elevated collar does contain dielectric material, it can also perform the function described earlier of reducing diversion of ion flux from the plasma to portions of the cathode outside the perimeter of the workpiece. In the conventional design shown in FIG. 1, the dielectric collar 30 extends axially above the surface of the wafer so as to combine the previously described functions of both an elevated collar and a dielectric shield.
While conventional elevated collars have been found to improve the spatial uniformity of semiconductor fabrication processes, further improvements in spatial uniformity would be desirable.